Hundreds of bison have left the boundaries of Yellowstone National Park this winter in search of food. The annual hunt for wild bison migrating from Yellowstone National Park has hit its highest level in decades. (ASSOCIATED PRESS / Ted S. Warren, file)

BILLINGS, Mont. (AP) — Hunters killed more wild bison migrating from Yellowstone National Park this season than they have in decades, with the numbers driven by strong participation from American Indians who harvest the animals under longstanding treaty rights.

Roughly 250 bison have been killed this season after leaving Yellowstone for low-elevation winter range in Montana. Combined with a mild winter, the means there’s unlikely to be a repeat this year of the wholesale slaughters that have killed thousands of bison in the last two decades in the name of disease control.

Tribal and state officials said Friday that should provide some relief for the park’s burgeoning bison herds, which leave Yellowstone in fewer number during mild winters.

Still, hunting carries its own challenges, beyond criticism from some animal advocates.

After scores of gut piles from harvested bison were found outside the park’s northern boundary near the town of Gardiner, wildlife officials removed 8,000 pounds of bison waste to avoid a situation in which the waste would attract grizzly bears now emerging from their winter dens.

In recent years, government agencies that oversee the bison have moved away from their past policy of capturing bison for slaughter or hazing them back into the park as soon as they cross the Montana boundary.

As a result, bison can again access tens of thousands of acres of historic grazing areas — and hunters have more chance to shoot them.

Since 2006, members of the Nez Perce have travelled to Montana to hunt bison under the terms of an 1855 government treaty that recognized the Yellowstone area as a traditional tribal hunting ground.

Several other tribes with treaty rights also participate. Combined the tribe’s members killed more than 200 bison this year.

For Lawrence, that’s much preferred to shipping bison to slaughter, which the tribe argues violates its rights by removing animals that hunters otherwise could harvest.

“We would like to see the population at a level where there’s an annual migration,” he said, adding that the tribe “is not interested in seeing a gross movement of animals” to slaughter.

A limited slaughter still is possible, officials said, and the U.S. Department of Agriculture is seeking up to 63 bison this year for use in an experimental animal contraception program.

Many bison carry the disease brucellosis. If transmitted to cattle, brucellosis can cause the animals to prematurely abort their calves.

Despite recent changes in federal policy that eased trade sanctions against states where brucellosis is found in cattle, Montana’s livestock industry and its supporters are pushing to restore restrictions that would keep bison in the park.

That includes so-called “zero tolerance” bison legislation pending before the Montana Legislature and a lawsuit that would reverse the state’s decision to allow the animals to roam largely free in the 75,000-acre Gardiner Basin.

The state is fighting the lawsuit and Montana Fish, Wildlife and Parks has come out against to the zero tolerance measure. Wildlife officials say stringent limits on where bison are allowed harkens back to the late 1980s, when the state encouraged hunters to kill every bison that crossed the Montana line.

That resulted in a record 489 bison killed in 1989, but also trigged an international outcry that led to the cancellation of bison hunting until it resumed in 2005.

Hunting is not allowed inside the park, so Yellowstone administrators rely on the killing of animals that migrate into Montana to keep the population in check. Officials set a target of removing 400 bison this year, from a population estimated at 4,200 animals before the hunting season.

The tribal hunts, conducted under long-standing treaty rights, are expected to continue through the end of March. State-licensed hunters took 37 bison during a season that ended Feb. 15.

“Our goal was to as much as possible manage the population level through hunting as opposed to other means,” said Pat Flowers, the Yellowstone region supervisor for Montana Fish, Wildlife and Parks. “If we can have a more consistent removal out of the park, we can get the population back down near the target of 3,000 to 3,500 bison.”